Turbomachines, such as those used in numerous aircraft applications, require a continuous flow of air through the machine to meet combustion process and cooling requirements.
Some turbomachines have an air inlet section in the form of a stator vane stage located in a forward area of the turbomachine. The vanes of this stage are positioned around the inlet area of the turbomachine like the spokes of a wheel. Each vane, which may have an airfoil cross-section, directs incoming air to a forward compressor section of the turbomachine.
The ambient air introduced to the turbomachine through its air inlet section may be characterized in terms of temperature, moisture content, and density. Icing of an air inlet structure may occur when the ambient temperature of in-flowing air is near freezing, or even a few degrees above freezing, in combination with a sufficient moisture content. A stator vane stage located in the air inlet section is one such icing-susceptible structure.
Icing may occur through a full range of aircraft and engine operating regimes, including operation in known icing conditions, as well as during periods of low power machine operation which may include an idle setting. For example, ice formation may occur during a descent maneuver when the engine is operating at idle, given the necessary conditions, absent an effective anti-icing system. In some turbomachines, throttling back to idle speed results in a reduction of temperature raise created by the rotors, with a correlating reduction in the pressure ratio of the air inlet section. Consequently, the surface temperature of stator blading suffers a correlated drop in temperature. The moisture contained in the impinging ambient air flow will freeze on the stator blades if the stator surface is not kept sufficient warm. While it is well know for icing to occur during engine idle operation, it is also know to occur at other power settings depending on ambient temperatures, flight speed, and moisture content of the inlet air. It is also well know for icing to occur in other flight regimes, such as takeoff or cruise, or even during apparently clear air low-humidity atmospheric conditions.
One anti-icing technique is to duct high temperature high pressure bleed air from a downstream machine section to the air inlet section. One problem with this technique is the consequential reduction of motive thrust by the turbomachine. Another problem with this technique is a correlated increase in the specific fuel consumption of the turbomachine during system operation due to the sacrificial airbleeding to the air inlet region. Yet another problem is the losses to the turbomachine due to the aerodynamically-comprising oversizing of the prior art stator vanes, whereby the oversizing was necessary to conduct and contain a sufficient volume of anti-icing bleed air.
A further problem with the prior practice is the need for a separate icing-condition sensor and related circuitry necessary to either alert the aircraft's crew to activate an anti-icing system or to automatically cycle the system as required.